Optical system comprising a spatial light modulator

Abstract

The invention concerns an optical system ( 10 ) comprising: a spatial light modulator ( 13 ) comprising an array of controllable elements adapted for generating a modulated light beam ( 17 ) by diffracting an incident light beam ( 12 ), the modulated light beam ( 17 ) comprising a modulated part and an unmodulated “zero order” part, the modulated part carrying an image to be projected into a replay volume ( 18 ), a control unit for applying a control signal to the controllable elements so as to control the modulated light beam ( 17 ) generated by the controllable elements, a perturbing optical element ( 30 ) for introducing optical aberrations in the incident light beam ( 12 ) and/or the modulated light beam ( 17 ) so as to spread the unmodulated “zero order” part of the modulated light beam in the replay volume ( 18 ), the perturbing optical element ( 30 ) generating distortions in the image to be projected, wherein the control signal comprises an image signal component carrying data representative of the image to be projected and a correction signal component representative of a correction pattern for correcting the distortions generated by the perturbing optical element ( 30 ) in the image to be projected.

The invention claimed is: 
     
       1. An optical system ( 10 ) comprising:
 a light emitting source ( 11 ) for generating an incident light beam ( 12 ); 
 a first spatial light modulator ( 13 ) comprising an array of controllable elements adapted for generating a modulated light beam ( 17 ) by diffracting the incident light beam ( 12 ), the modulated light beam ( 17 ) comprising a modulated part and an unmodulated “zero order” part, the modulated part carrying an image to be projected into a replay volume ( 18 ); 
 a control unit ( 20 ) for applying a control signal to the controllable elements so as to control the modulated light beam ( 17 ) generated by the controllable elements, wherein the optical system ( 10 ) further comprises a perturbing aberrant optical element ( 30 ) for introducing optical aberrations in the incident light beam ( 12 ) and/or the modulated light beam ( 17 ), so as to spread the unmodulated “zero order” part of the modulated light beam ( 17 ) in the replay volume ( 18 ) where the image carried by the modulated part is projected, with a maximum intensity of the unmodulated “zero order” part in the replay volume ( 18 ) being less than a maximum intensity of the modulated part in all the replay volume ( 18 ) where the image carried by the modulated part is projected, wherein the perturbing aberrant optical element ( 30 ) generates distortions in the image carried by the modulated part to be projected; and 
 a second spatial light modulator, or 
 at least one cylindrical lens having a cylindrical surface, for spreading the incident light beam ( 12 ) and/or the modulated light beam ( 17 ) according to a spreading direction, wherein the control signal comprises an image signal component carrying data representative of the image to be projected and a correction signal component representative of a correction pattern for correcting the distortions generated by the perturbing aberrant optical element ( 30 ) in the image carried by the modulated part to be projected, the optical system presenting a focal plane only for the modulated part of the modulated light beam, and not for the unmodulated “zero order” part, after correction by the first spatial light modulator ( 13 ). 
 
     
     
       2. The optical system according to  claim 1 , wherein, for a same power of the unmodulated “zero order” part in the replay volume, the perturbing aberrant optical element ( 30 ) decreases, by a factor ten or more, a maximum intensity of the unmodulated “zero order” part of the modulated light beam ( 17 ) in the replay volume ( 18 ). 
     
     
       3. The optical system according to  claim 1 , further comprising a support ( 19 ) for receiving a sample, and an objective lens assembly ( 21 ) comprising an objective lens ( 25 ) and additional lenses ( 22 ,  24 ) for projecting the image into the sample, wherein: typical dimensions ΔX, ΔY and ΔZ of the replay volume are ΔX=ΔY=λ×f 1 ×f obj /(a×f 2 ) for dimensions transverse to an optical axis of the objective lens and the additional lenses and ΔZ=λ×f 1 ×f obj /(2×NA×a×f 2 ) the dimension along the optical axis, where f 1  and f 2  are the focal lengths of the additional lenses ( 22 ,  24 ) situated downstream from the perturbing aberrant optical element ( 30 ), f obj  is the focal length of the objective lens ( 25 ) situated downstream from the additional lenses ( 22 ,  24 ), NA is the numerical aperture of the objective lens ( 25 ), and a is the typical size of phase actuators of the first spatial light modulator ( 13 ). 
     
     
       4. The optical system according to  claim 3 , wherein the aberrations introduced by the perturbing aberrant optical element ( 30 ) have spatial frequencies that are less than or equal to 1/(2a), where a is a period of the array of controllable elements. 
     
     
       5. The optical system according to  claim 1 , wherein the first spatial light modulator ( 13 ) is a spatial phase modulator. 
     
     
       6. The optical system according to  claim 1 , wherein the perturbing aberrant optical element ( 30 ) comprises a first cylindrical lens having a first cylindrical surface and a second cylindrical lens having a second cylindrical surface, for spreading the incident light beam ( 12 ) and/or the modulated light beam ( 17 ) according to a first spreading direction and to a second spreading direction defining an angle relative to the first spreading direction. 
     
     
       7. The optical system according to  claim 1 , further comprising a support ( 19 ) and a light sensitive substance located in the replay volume ( 18 ) in the support ( 19 ), wherein:
 the modulated part carries the image to be projected into the replay volume ( 18 ) for exciting the light sensitive substance, and 
 the light sensitive substance exhibits a non-linear optical response and is only excited by the modulated part of the modulated light beam ( 17 ). 
 
     
     
       8. The optical system according to  claim 1 , further comprising a dispersive optical element ( 35 ) for pulse stretching the modulated light beam, and an optical arrangement ( 36 ) for pulse compressing the stretched modulated light beam in the replay volume ( 18 ). 
     
     
       9. The optical system according to  claim 1 , further comprising an optical fiber or an optical fiber bundle ( 37 ), wherein the image is projected in the replay volume ( 18 ) through the optical fiber or optical fiber bundle. 
     
     
       10. The optical system according to  claim 1 , further comprising an amplitude modulator ( 38 ) arranged between the first spatial light modulator ( 13 ) and a sensitive substance, the amplitude light modulator modulating an amplitude of the modulated light beam, preferably increasing a modulation speed. 
     
     
       11. Method for spreading an unmodulated “zero-order” part of a modulated light beam ( 17 ) generated by a first spatial light modulator ( 13 ) with the optical system defined in  claim 1 , comprising the steps of:
 a) generating an incident light beam ( 12 ); 
 b) diffracting the incident light beam ( 12 ) by means of the spatial light modulator ( 13 ); and 
 c) generating the modulated light beam ( 17 ) by applying a control signal to controllable elements of the first spatial light modulator ( 13 ), the control signal comprising an image signal component carrying data representative of an image to be projected, the modulated light beam ( 17 ) comprising a modulated part and the unmodulated “zero-order” part, the modulated part carrying the image to be projected into a replay volume ( 18 ); 
 wherein optical aberrations are introduced in the incident light beam ( 12 ) and/or the modulated light beam ( 17 ) by means of a perturbing aberrant optical element ( 30 ) so as to spread the unmodulated “zero-order” part of the modulated light beam in the replay volume ( 18 ) where the image is projected, with a maximum intensity of the unmodulated “zero order” part in the replay volume ( 18 ) being less than a maximum intensity of the modulated part in all the replay volume ( 18 ); 
 the perturbing aberrant optical element generating distortions in the image carried by the modulated part to be projected, and comprising:
 a second spatial light modulator, or 
 at least one cylindrical lens having a cylindrical surface, for spreading the incident light beam and/or the modulated light beam according to a spreading direction, 
 
 distortions generated by the perturbing aberrant optical element ( 30 ) in the image to be projected are corrected by applying to the controllable elements a correction signal component of the control signal representative of a correction pattern, the optical system presenting a focal plane only for the modulated part of the modulated light beam, and not for the unmodulated “zero order” part, after correction by the first spatial light modulator ( 13 ). 
 
     
     
       12. Method according to  claim 11 , wherein the perturbing aberrant optical element ( 30 ) comprises a first cylindrical lens having and a second cylindrical lens, and the spatial light modulator does not shift away the image of the modulated part of the modulated light beam from the image of the unmodulated “zero order” part. 
     
     
       13. Method according to  claim 11 , wherein the modulated part carries the image to be projected into the replay volume ( 18 ) for exciting a light sensitive substance located in the replay volume ( 18 ) in a support ( 19 ). 
     
     
       14. Method according to  claim 11 , wherein:
 the modulated part carries the image to be projected into the replay volume ( 18 ) for exciting a light sensitive substance located in the replay volume ( 18 ), and 
 the light sensitive substance is a fluorescent material which exhibits a non-linear optical response and being only excited by the modulated part of the modulated light beam ( 17 ). 
 
     
     
       15. The optical system according to  claim 14 , wherein the first cylindrical lens and the second cylindrical lens having different vergences, the first spatial light modulator ( 13 ) does not shift away the image of the modulated part of the modulated light beam from the image of the unmodulated “zero order” part. 
     
     
       16. Method according to  claim 11 , wherein, for a same power of the unmodulated “zero order” part in the replay volume, the perturbing aberrant optical element ( 30 ) decreases, by a factor ten or more, a maximum intensity of the unmodulated “zero order” part of the modulated light beam ( 17 ) in the replay volume ( 18 ). 
     
     
       17. Method according to  claim 11 , wherein the optical system comprises a support ( 19 ) for receiving a sample, and an objective lens assembly ( 21 ) comprising an objective lens ( 25 ) and additional lenses ( 22 ,  24 ) for projecting the image into the sample, wherein typical dimensions ΔX, ΔY and ΔZ of the replay volume are ΔX=ΔY=λ[×f]_1 [×f]_obj/(a[×f]_2) for dimensions transverse to an optical axis of the objective lens and the additional lenses and ΔZ=(λ[×f]_1 [×f]_obj)/((2×NA×a[×f]_2)) for the dimension along the optical axis, where: f_1 and f_2 are the focal lengths of the additional lenses ( 22 ,  24 ) situated downstream from the perturbing aberrant optical element ( 30 ), f_obj is the focal length of the objective lens ( 25 ) situated downstream from the additional lenses ( 22 ,  24 ), NA is the numerical aperture of the objective lens ( 25 ) and a is the typical size of phase actuators of the first spatial light modulator ( 13 ).